1. Field of the Invention
The present invention relates to a production method for a magnetic carrier that is used in a development method where an electrostatic latent image formed on an electrostatic latent image bearing member is developed with a two-component type developer to form a toner image on the electrostatic latent image bearing member.
2. Description of the Related Art
Recently, a two-component type developer used in electrophotography has been required to be higher in image quality and stability in its performance from the viewpoint of satisfying market needs such as accelerated color shift in office use, higher definition corresponding to a graphic market and higher speed corresponding to light printing.
At present time, as a magnetic carrier constituting a two-component type developer, a magnetic carrier where a surface of ferrite core particles or magnetic material-dispersed resin core particles (hereinafter, referred to as magnetic carrier core particles) is formed from a coated layer of a resin composition is mainly used.
The coated layer plays the role of controlling charge injection from a developer bearing member via the magnetic carrier to a photosensitive member to stabilize a charge amount distribution of toner and to improve endurance capable of stably imparting charges also for use in a long period of time.
So far, as a method of coating a surface of the magnetic carrier core particles with a resin composition, a so-called wet coating process has been mainly used.
As a wet coating process, a process where a surface of magnetic carrier core particles floating in a fluid layer is spray-coated with a coating solution obtained by dissolving a resin composition in a solvent, or a process where magnetic carrier core particles are dipped in a coating solution where the resin composition is dissolved in a solvent can be cited.
The wet coating process is effective from the viewpoint of uniformly coating a surface of magnetic carrier core particles with a resin composition. However, a problem of the wet coating process is that when a solvent vaporizes magnetic carriers tend to coalesce.
Even when coalesced magnetic carrier is disintegrated by stirring, a surface of the magnetic carrier core particles is exposed on the disintegrated surface thereof to tend to cause a so-called charge injection phenomenon that is an injection phenomenon of charges from the magnetic carrier to a photosensitive member.
When the charge injection phenomenon occurs, a surface potential of a photosensitive member converges on a development bias to be unable to secure a development contrast to result in occurrence of, in some cases, a blank image.
Furthermore, when a surface of the magnetic carrier core particles is exposed, in particular under high temperature and high humidity, toner charges cannot be retained and thereby, owing to low toner charge after leaving for a long period of time, image defects such as fogging tend to occur.
Still furthermore, in order to completely remove a solvent, a drying step is separately necessary to cause a factor of tact-up; accordingly, the wet coating process yet has many improvement points also from the production point of view.
In this connection, as a process that overcomes the problem of the wet coating process, a process that conducts a dry coating by heat has been proposed.
For example, Japanese Patent Application Laid-Open No. 2009-160307 discloses a process where a powdery material to be processed is, under mixing and stirring with a stirring blade by use of a high-speed stirring mixer, thermally coated at a temperature higher than the glass transition temperature (Tg) of resin composition particles contained in the material to be processed or more to obtain a magnetic carrier.
According to the process, a heat medium is flowed to a jacket disposed inside of a body casing to heat an entire apparatus to raise a temperature of the entire material to be processed to the glass transition temperature (Tg) or more of resin composition particles contained in the material to be processed.
The above-mentioned process is advantageous from the viewpoint that there is no need of separately disposing a drying step. However, since a temperature of an entire material to be processed is raised to the glass transition temperature (Tg) of the resin composition particles contained in the material to be processed or more, the magnetic carrier tends to unify; accordingly, there is still room for improvement in uniform coating.
On the other hand, a process where dry coating is conducted by a mechanical impact force has been proposed.
For example, Japanese Patent Application Laid-Open No. S63-235959 proposes a process where a surface modifying apparatus having a rotor and a stator is used to coat a surface of magnetic carrier core particles with resin composition particles having a particle diameter of one tenth or less the magnetic carrier core particles.
According to the above-mentioned process, by the use of a separate apparatus from an apparatus for coating, the resin composition particles are dispersed on a surface of the magnetic carrier core particles; accordingly, there is an inconvenience in that a dispersion apparatus is separately necessary.
When the dispersion apparatus is not used, the resin composition particles remain isolated to result in difficulty in well coating a surface of the magnetic carrier core particles with the resin composition particles.
Furthermore, even when the resin composition particles are attached onto a surface of the magnetic carrier core particles by the use of an apparatus separate from an apparatus for coating, in the case where the resin composition particles are much added, non-attached resin composition particles remain isolated; accordingly, uniform coating is difficult. Still furthermore, since a coated layer in a formulated amount is not formed, variation in charge-imparting property is caused between magnetic carrier particles or lots. Hereinafter, the resin composition particles that could not be attached are referred to as residual resin composition particles.
Accordingly, a coating amount of the resin composition particles is limited by the method, and, thereby, in some cases, toner charge amount control or charge injection control from a magnetic carrier to a photosensitive member become difficult.
On the other hand, in order to increase a coating amount of resin composition particles, in Japanese Patent No. 2811079, a process where, by the use of a high-speed stirring mixer, the resin composition particles are intermittently supplied by dividing into at least two times has been disclosed.
However, also in the process, the residual resin composition particles that were not used for coating are generated, every time when magnetic carriers are produced, performance such as a charging property or specific resistance of the magnetic carrier fluctuates between magnetic carriers; accordingly, in some cases, a magnetic carrier stable over a long period of time cannot be obtained.
Furthermore, as a separate composite coating apparatus that uses a mechanical impact force, in Japanese Patent Application Laid-Open No. 2005-270955, a coating apparatus has been proposed.
It is said that the coating apparatus, while taking an advantage of a rotary blade apparatus, applies an unprecedentedly strong force to a material to be processed such as a powder or the like to heighten a stirring effect, and, thereby, various processings such as compositing, surface modification and the like of a material to be processed such as powder and the like can be conducted.
However, depending on properties of particles used as resin composition particles, there is a problem that residual resin composition particles are generated. Furthermore, in order to obtain a magnetic carrier where the occurrence of cracks or chips on a surface of a magnetic carrier is suppressed, a coating amount of the resin composition particles is much and uniform coating is conducted, there still remains room for improvement. In particular, there is much room for improvement in uniformity of the magnetic carrier surface.